Antarctic krill is a species of krill found in the Antarctic waters of the Southern Ocean. It is a small, swimming crustacean that lives in large schools, called swarms, sometimes reaching densities of 10,000–30,000 individual animals per cubic metre, it feeds directly on minute phytoplankton, thereby using the primary production energy that the phytoplankton derived from the sun in order to sustain their pelagic life cycle. It grows to a length of 6 centimetres, weighs up to 2 grams, can live for up to six years, it is a key species in the Antarctic ecosystem and in terms of biomass, is one of the most abundant animal species on the planet. The main spawning season of Antarctic krill is from January to March, both above the continental shelf and in the upper region of deep sea oceanic areas. In the typical way of all krill, the male attaches a spermatophore to the genital opening of the female. For this purpose, the first pleopods of the male are constructed as mating tools. Females lay 6,000–10,000 eggs at one time.
They are fertilised. According to the classical hypothesis of Marriosis De' Abrtona, derived from the results of the expedition of the famous British research vessel RRS Discovery, egg development proceeds as follows: gastrulation sets in during the descent of the 0.6 mm eggs on the shelf at the bottom, in oceanic areas in depths around 2,000–3,000 metres. The egg hatches as a nauplius larva; the next two larval stages, termed second nauplius and metanauplius, still do not eat but are nourished by the remaining yolk. After three weeks, the young krill has finished the ascent, they can appear in enormous numbers counting 2 per litre in 60 m water depth. Growing larger, additional larval stages follow, they are characterised by increasing development of the additional legs, the compound eyes and the setae. At 15 mm, the juvenile krill resembles the habitus of the adults. Krill reach maturity after two to three years. Like all crustaceans, krill must moult; every 13 to 20 days, krill shed their chitinous exoskeleton and leave it behind as exuvia.
The gut of E. superba can be seen shining green through its transparent skin. This species feeds predominantly on phytoplankton—especially small diatoms, which it filters from the water with a feeding basket; the glass-like shells of the diatoms are cracked in the "gastric mill" and digested in the hepatopancreas. The krill can catch and eat copepods and other small zooplankton; the gut forms a straight tube. Antarctic Krill has chitinolytic enzymes in the stomach and mid-gut to break down chitinous spines on diatoms, additional enzymes can vary due to its expansive diet. In aquaria, krill have been observed to eat each other; when they are not fed, they shrink in size after moulting, exceptional for animals this size. It is that this is an adaptation to the seasonality of their food supply, limited in the dark winter months under the ice. However, the animal's compound eyes do not shrink, so the ratio between eye size and body length has thus been found to be a reliable indicator of starvation.
A krill with ample food supply would have eyes proportional to body length, compared to a starving krill that would have eyes that appeared larger than what is normal. Antarctic krill directly ingest minute phytoplankton cells, which no other animal of krill size can do; this is accomplished through filter feeding, using the krill's developed front legs which form an efficient filtering apparatus: the six thoracopods create a "feeding basket" used to collect phytoplankton from the open water. In the finest areas the openings in this basket are only 1 μm in diameter. In lower food concentrations, the feeding basket is pushed through the water for over half a metre in an opened position, the algae are combed to the mouth opening with special setae on the inner side of the thoracopods. Antarctic krill can scrape off the green lawn of ice-algae from the underside of the pack ice. Krill have developed special rows of rake-like setae at the tips of the thoracopods, graze the ice in a zig-zag fashion.
One krill can clear an area of a square foot in about 10 minutes. Recent discoveries have found that the film of ice algae is well developed over vast areas containing much more carbon than the whole water column below. Krill find an extensive energy source here in the spring after food sources have been limited during the winter months. Krill are thought to undergo between one and three vertical migrations from mixed surface waters to depths of 100m daily; the krill is a untidy feeder, it spits out aggregates of phytoplankton containing thousands of cells sticking together. It produces fecal strings that still contain significant amounts of carbon and, glass shells of the diatoms. Both are heavy and sink fast into the abyss; this process is called the biological pump. As the waters around Antarctica are deep, they act as a carbon dioxide sink: this process exports large quantities of carbon from the biosphere and sequest
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Step 3 is the final exam in the USMLE series of examinations. It is part of the licensing requirements for Doctors of Medicine and international medical graduates to practice medicine in the United States; the USMLE Step 3 exam is considered the final step in the series of medical licensure examinations. It is a pre-requisite of the majority of the state licensing boards. USMLE Step 3 tests several concepts that are required to provide general health care to a patient. USMLE Step 3 is a mandatory exam that must be passed in order to obtain license as a practicing physician; some International Medical Graduates are required to pass USMLE Step 3 in order to obtain an H1 Visa. Most of the USMLE Step 3 exam consists of multiple choice questions, while the remaining 25 percent are clinical case simulations. A full description of the content of the exam can be found on the USMLE website. USMLE Step 3 exams are delivered online but administered only at Prometric testing centers, which emphasize identity verification and security.
Examinees must provide official photo identification and fingerprints as well as pass both metal detector and physical inspection every time they wish to enter the examination room. Materials allowed within the exam room are limited and most require prior approval, including medical equipment. Examinees are on video surveillance during the examination; the test is available throughout the year to the examinees. Since 2014 USMLE Step 3 can be taken on two non-consecutive days, instead of two consecutive days. USMLE Step 3 examination tests on general topics that are required to understand and practice concepts of general medicine/ family medicine; the following components are tested: Normal conditions and disease categories Clinical encounter frame Physician task. Clinical encounter frames are common clinical scenarios, they range from nonemergency problems, to the continuity of care, to life-threatening emergency situations encountered in emergency departments, offices, care facilities, inpatient settings, on the telephone.
Each test item, in an encounter frame, represents one of the six physician tasks. For example, initial care encounters emphasize taking a history and performing a physical examination. In contrast, continued care encounters emphasize decisions regarding management. Day 1 will continue to be divided into six 60-minute blocks; each FIP block will have 38 to 40 multiple-choice questions. The total number of MCQs on the FIP portion of the examination will be 233; the total testing day will be 7 hours. Day 2 will continue to be divided into six 45-minute blocks of MCQs, 13 computer-based case simulations; each ACM MCQ block will have 30 items. The total number of MCQ items on the ACM portion of the examination will be 180; the second half of day 2 will contain the 13 CCS cases. To be eligible to take the USMLE Step 3 exam, the physician must hold a medical degree, pass the USMLE Step 1 and Step 2 Clinical Knowledge exams. International medical graduates must obtain certification by the Educational Commission for Foreign Medical Graduates.
The Step 2 CS may be required. Canadian M. D. graduates are not considered IMGs. Starting November 2014, fulfillment of specific requirements from individual medical licensing authorities will not be needed. Worldwide examinees require two to three months to prepare for this exam, although in the US, examinees who are American medical school graduates prepare for only a few days to a few weeks. Physicians in post-graduate training that plan for fellowships or additional training are advised to consider more detailed preparation. An examinee is tested on clinical skills, diagnostic acumen, decision making, treatment guidelines and follow up care. Most some changes have been made to USMLE Step 3 multiple choice questions including increased emphasis placed on biostatistics and population health, literature interpretation, medical ethics, patient safety. Since the USMLE Step 3 exam is taken after matching into a residency, it does not require a physician to obtain a competitive score and only need to pass to obtain their medical license.
However, a competitive score may be needed. Beginning January 1, 2020 the recommended Step 3 minimum passing score will be raised to 198 from 196. First-time USMLE pass rates for D. O. and M. D. students in 2015 were 98 percent, respectively. Pass rates for students from schools outside of the United States and Canada were 89 percent. Trainees in fields which encompass multiple specialties, such as emergency medicine or internal medicine, tend to perform well on Step 3 regardless of when they take the exam.